1. Field of the Invention
The present invention relates to differential-pressure flow measurement and more particularly to differential pressure transducers capable of converting differential pressure into an electrical signal.
It is general practice to measure fluid flow rate by inserting into the flow pipe a restrictive means, such as an orifice, to obtain a pressure differential across the restrictive means corresponding to the flow rate of the fluid passing through the pipe. This differential pressure may be applied to a differential-pressure transducer to cause a displacement of means, such as a diaphragm, which displacement can be detected by electrical devices and converted into measurable electrical signals proportional to the displacement and hence to the differential pressure and fluid flow rate.
2. Description of the Prior Art
FIG. 1 of the within drawings illustrates a prior-art differential-pressure transducer of a type such as is disclosed for example in U.S. Pat. No. 3,618,390. The instrument of FIG. 1 includes a measuring diaphragm 1 mounted in a generally circular cross-section body 2 which has internal chambers 21 and 22 and may be formed of two body sections corresponding to the two chambers. Stretched seal diaphragms 31 and 32 are welded to the body at 33 and 34, together with first and second connecting members 41 and 42, on both sides of the body. The seal diaphragms 31 and 32 form isolation chambers 35 and 36 which communicate with the internal chambers 21 and 22 through passageways 23 and 24 respectively. The spaces formed therein are filled with a fluid such as silicone oil.
The body 2 with associated elements is installed as a unit in an internal space 51 in a housing 5. A third connecting member 52 integral with the housing 5 is welded to the second connecting member 42 at 53. A fourth connecting member 54 in the form of an annular plate is welded to the first connecting member 41 and housing 5 at 55 and 56. Covers 71 and 72 are fastened to the housing 5 on both sides, respectively, with bolts 81 and nuts 82. O-rings 91 and 92 are mounted between the first connecting member 41 and the cover 71 and between the second connecting member 42 and the cover 72 respectively. The O-rings 91 and 92, seal diaphragms 31 and 32, first and second connecting members 41 and 42, and covers 71 and 72 form chambers 73 and 74 respectively, into which the flowing fluid is introduced.
The pressures of the fluid in the chambers 73 and 74 act on the measuring diaphragm 1 from both sides via the seal diaphragms 31 and 32 and the seal fluid, causing the measuring diaphragm 1 to be displaced by the differential pressure. By detecting this displacement, the differential pressure can be measured. In the event an overpressure is applied, the measuring diaphragm 1 will bottom against the concave wall of either chamber 21 or 22 to obviate error in measurement.
In this type of device, the measuring diaphragm 1 and the body 2 are made of an elastic material, ordinarily the same for both. The housing 5 and the third and fourth connecting members 52 and 54 are made of carbon steel whose expansion coefficient is roughly similar to that of the diaphragm and body material. This serves to prevent the tension of the measuring diaphragm 1 from being altered by changes in temperature, thereby avoiding span error. The seal diaphragms 31 and 32, first and second connecting members 41 and 42, and covers 71 and 72 are of stainless steel, and the O-rings 91 and 92 are of a resilient material such as Teflon system rubber material to allow these elements to have high resistance to corrosion by the fluid being measured. The connecting members 41, 42, 52 and 54 located between the body 2 and the housing 5 allow a certain amount of deflection which compensates for variations in the tightening force of bolts 81 and nuts 82 to prevent the tension of the measuring diaphragm 1 from being altered.
Teflon system rubber materials in general are vulnerable to strongly corrosive chemicals such as hydrochloric acid. When the fluid being measured is hydrochloric acid or the like, therefore, the O-rings 91 and 92 will be replaced with silicon gaskets or metal O-rings. In practice, however, a great pressure must be applied to such elements to maintain substantial seal effect, with the result that a great force is exerted between the first and second connecting members 41 and 42 and covers 71 and 72. Consequently, variations in the force with which the covers 71 and 72 are fastened in position directly affect the force with which the body 2 is fastened in position, causing the tension of the measuring diaphragm 1 to be varied. In such construction, therefore, the connecting means 41, 42, 52 and 54 do not serve the needed function. Furthermore, because the covers 71 and 72, made of stainless steel whose expansion coefficient is considerably different from that of the elastic material used for the body 2, are connected axially to the body 2, the force with which the body 2 is fastened in position varies with changes in temperature, causing the tension of the measuring diaphragm 1 to be varied, so that span error will result.